Copyright
The University of Utah 2001
Updated February 2007
Updated September 2007
Updated September 2008
Updated September 2009
Updated September 2010
Updated November 2012
Updated September 2013
Updated January 2015
Updated August 2016

Major Events
in Brain Development
Brain and spinal
cord development begins with formation of the neural tube closure, which
is completed by 28 days of gestation. Prosencephalization then occurs
to
form the cerebral
hemispheres and diencephalon. At 2 to 4 months gestation, there is neuronal
proliferation followed by migration at 3 to 5 months. Formation of the
gyri begins with the major fissures being formed in the 5th month, the
secondary sulci by 7 months, and the tertiary sulci by 9 months gestation.
Finally, neuronal organization and myelination starts in the 3rd trimester
and continues on well after birth. Organization and myelination are the
major processes that account for brain growth and maturation.

Brain Growth
The newborn brain weighs 400 grams. At one year of age the brain weighs
1,000 grams. By 2 years of age the brain has reached 80 percent of its
adult size. By 18 years of age the brain has reached its adult weight of
1400 grams.

Neuronal Growth
and Organization
By 36 weeks gestation, the process of neuronal proliferation is nearly
complete. Although there is some limited proliferation of neurons throughout
life, their contribution to brain growth is negligible. Growth of the
individual neurons plus the elaboration and proliferation of dendritic
and axonal
processes and connections is responsible for most of the brain’s
growth.
The diameter of a neuroblast is 5 microns while a mature neuron
can measure 50 microns, a 1,000-fold increase in cell volume. Increase
in the size of glial cells also occurs. During the organization process
of brain development there is increasing dendritic arborization, synaptogenesis,
and axonal connections, which make a significant contribution to brain
growth as well as the maturation and development of brain function.

Myelination and
Development
The next developmental process
that greatly contributes to brain growth and maturation is myelination.
Myelination begins early in the 3rd trimester with the most rapid period
of myelination occurring in the first two years of life. The myelination
process follows a specific time course and pattern. The acquisition of
neurodevelopmental milestones parallels and reflects this pattern. Myelination
occurs early for motor-sensory roots, special senses and the brainstem;
those structures necessary for reflex behavior and survival.
The corticospinal
tract starts to myelinate at 36 weeks gestation and myelination is completed
by the end of the 2nd year of life. Myelination of the corticospinal tract
begins at the proximal portion of the axon and the shortest axons are the
first to myelinate. The axons for the upper extremities and the trunk myelinate
next. The axons for the lower extremities, which are the longest axons,
are the last to myelinate with the process being completed by 24 months
of age. This myelination pattern correlates with the progressive head-to-toe
acquisition of developmental milestones.
The motor patterns of the immature
brain are predominately under brainstem control and lack cortical modulation
until there is the necessary maturation of these higher brain center connections.

Cerebral Hemisphere
Myelination
The areas of the cerebral hemispheres that are first to myelinate are
the posterior portion of the frontal lobes, the parietal lobes and
areas of
the occipital lobes. The frontal and temporal lobes then myelinate and
by the end of the second year, myelination of the cerebrum is largely
completed, although there are interconnections of the association
cortex that are
still being myelinated into the 2nd and 3rd decades of life.